The scintillation camera has been the standard single photon imaging device in nuclear medicine for more than 25 years. This device, originally invented by Hal O. Anger in the late 1950's, has been continuously refined and improved since it was first developed. Contemporary scintillation cameras exhibit high levels of performance as compared to earlier models.
More recently, scintillation imaging systems have been adapted to single photon emission computed tomography (SPECT). This procedure usually involves rotating one or more scintillation cameras around a subject to obtain projection images which are required for tomographic reconstruction of the internal radioactivity distribution. These systems are now in use for both planar projection imaging and for SPECT in nuclear medicine clinics worldwide.
For about the last decade or so an alternative method to single photon imaging with the scintillation camera has received increasing academic and commercial attention, positron emission tomography (PET). According to this technique, positron emitting radioisotopes are attached to a desired tracer compound, administered to the patient and then imaged tomographically. The annihilation of a positron by a normal electron in the body gives rise to two simultaneous 511 keV gamma rays traveling nearly directly away from each other along the same line. Simultaneous detection of these two gamma rays (usually) by rings of detectors around the body defines the projection line along which the annihilation site must lie. The set of all such lines can be sorted into projection sets and these sets reconstructed tomographically to yield the internal activity distribution that gave rise to the projection sets. PET systems based on this "ring" design are considered to be the current state of the art.
Despite the sophistication of this design, ring machines possess a number of limitations: spatial resolutions better than 2 mm are difficult to achieve, thereby precluding use of such systems in small animal studies or in high resolution human studies; spatial resolution varies appreciably from the center to the edge of the field of view due to a depth of interaction effect and so on. Of perhaps greater importance, PET ring scanners are not capable of single photon imaging. On the other hand, scintillation cameras of conventional design are generally believed to be incapable of effective PET imaging. Thus, at present, there is no system believed capable of both forms of imaging.
The present invention is directed to an imaging system that is capable of both single photon imaging and positron emission tomography.